Canon EOS 90D Advanced User's Manual page 116

Setting the Aperture Value Before Shooting (Aperture-Priority AE)
Checking the Area in Focus M
The larger the aperture value, the wider the area (from the foreground to the
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background) in focus, but the darker the viewfi nder display.
The depth-of-fi eld eff ect is readily apparent on the Live View image as
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you change the aperture value and press the depth-of-fi eld preview button
( = 71).
Exposure is locked (AE lock) as you hold down the depth-of-fi eld preview
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button.
Pressing the depth-of-fi eld preview button when a Speedlite 470EX-AI is
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attached with the < X > switch set to < Z > (full-auto) starts AI.B full-auto
distance measurement.
116
Press the depth-of-fi eld preview button to
stop down the lens to the current aperture
setting and check the area in focus ( depth
of fi eld).